Adding Content to the Admin Edit Screen
When creating custom post types, you often need custom fields for them. In the past, your options were “normal” or “side” with priorities. Depending on what else is displayed on your edit screen, these boxes could vary in where they appeared.
And sometimes you just need to add a comment or note to the edit pages.
Good News! We now have new hooks to place them precisely:
Here’s an article that explains the usage: https://make.wordpress.org/core/2012/12/01/more-hooks-on-the-edit-screen/